Field of the Invention
The present invention relates to a correction method for an image forming apparatus, for correcting distortion and uneven image density of an image during image formation of a two-dimensional image by the image forming apparatus, e.g., a digital copying machine, a multifunctional peripheral, or a laser printer.
Description of the Related Art
In electrophotographic image forming apparatus such as a laser printer and a copying machine, there has been generally known a configuration to form a latent image on a photosensitive member with use of a light scanning device configured to perform scanning with a laser beam. In the light scanning device of a laser scanning type, a laser beam collimated with use of a collimator lens is deflected by a rotary polygon mirror, and the deflected laser beam is formed into an image on a photosensitive member with use of an elongated fθ lens. Further, there is known multibeam scanning in which a laser light source having a plurality of light emitting points is included in one package so as to perform scanning with a plurality of laser beams simultaneously.
Meanwhile, in order to form a satisfactory image without uneven image density and banding, it is desired that distances between scanning lines of which positions to be scanned with a laser beam are adjacent to each other in a rotational direction of the photosensitive member be equal to each other. However, the distances between the scanning lines are varied due to a plurality of factors described below. The distances between the scanning lines on the photosensitive member are varied by, for example, a fluctuation in a surface speed of the photosensitive member, or a rotation speed fluctuation of a rotary polygon mirror. Further, the distances between the scanning lines are also varied by a variation in angle of mirror faces of the rotary polygon mirror with respect to a rotary shaft of the rotary polygon mirror and a variation in intervals between light emitting points arranged on a laser light source. FIG. 19A is an illustration of a state in which an interval between the scanning lines is varied periodically, with scanning of laser beams being represented by horizontal lines. As illustrated in FIG. 19A, when the interval between the scanning lines of laser beams is small, an image is developed darkly. When the interval between the scanning lines of laser beams is large, an image is developed lightly. Thus, the development is liable to be detected as moire and the like. To cope with uneven image density and banding caused by such factors, there has been proposed a technology of correcting banding by controlling an exposure amount of the light scanning device. For example, in Japanese Patent Application Laid-Open No. 2012-098622, there is described a configuration in which a beam position detection unit configured to detect a beam position in a sub-scanning direction is arranged in the vicinity of the photosensitive member, and the exposure amount of the light scanning device is adjusted based on scanning distance information obtained from a detected beam position, to thereby make banding less noticeable.
However, in the conventional method of adjusting density based on the exposure amount, an optimum amount of controlling a light amount is varied depending on a change in image forming conditions of the image forming apparatus. Therefore, it is difficult to perform banding correction stably. As the changes in image forming conditions, there are given, for example, a change in ambient temperature environment of the image forming apparatus, a change in sensitivity to light of the photosensitive member, and a change with time of characteristics of a toner material.
Further, in a color image forming apparatus, when positional deviation occurs at a relatively long period, positional deviation occurs between colors at a long period to cause an image defect, e.g., uneven hue. FIG. 19B is an illustration of a state of positional deviation of each scanning line. When printing is performed at a resolution of 1,200 dpi (scanning line interval: 21.16 μm) with respect to an image having an image width of 297 mm in an A4 longitudinal direction, about 14,000 scanning lines are formed. Due to the above-mentioned factors, e.g., a fluctuation in surface speed of the photosensitive member, the positional deviation amount between an ideal position of the scanning line and an actual scanning position in an image area is varied in a non-uniform manner. In FIG. 19B, in the 2,000th line and 7,000th line from a leading edge of an image, the scanning position of a scanning line represented by the solid line is deviated in a front direction from an ideal position represented by the broken line, and in the 10,000th line, the scanning position is deviated in a direction opposite to the front direction. Thus, when the scanning line, that is, the image position is deviated from the ideal position in the image area, a problem, e.g., a hue variation occurs, and hence a configuration to move the absolute position of image data is required.